Evolution of the high-mass end of the stellar initial mass functions in starburst galaxies

TL;DR

This study uses chemodynamical simulations to explore how the high-mass end of the stellar initial mass function varies over time and space in starburst galaxies, revealing a trend towards more top-heavy IMFs in regions with higher star formation rates.

Contribution

It introduces a model linking IMF slope evolution to local density and metallicity, and predicts spatial and temporal variations in the IMF in starburst galaxies, aligning with recent observations.

Findings

IMF slope evolves from Salpeter to top-heavy in starburst galaxies.

IMF slope correlates with star formation rate density.

Inner regions of starburst galaxies have more top-heavy IMFs.

Abstract

We investigate the time evolution and spatial variation of the stellar initial mass function (IMF) in star-forming disk galaxies by using chemodynamical simulations with an IMF model depending both on local densities and metallicities ([Fe/H]) of the interstellar medium (ISM). We find that the slope (alpha) of a power-law IMF (N(m) ~ m^-alpha) for stellar masses larger than 1M_sun evolves from the canonical Salpeter IMF (alpha ~ 2.35) to be moderately top-heavy one (alpha ~ 1.9) in the simulated disk galaxies with starbursts triggered by galaxy interaction. We also find that alpha in star-forming regions correlates with star formation rate densities (Sigma_SFR in units of M_sun yr^{-1} kpc^{-2}). Feedback effects of Type Ia and II supernovae are found to prevent IMFs from being too top-heavy (alpha < 1.5). The simulation predicts alpha ~ 0.23 log Sigma_SFR + 1.7 for log Sigma_SFR > -2 (i.e., more top-heavy in higher Sigma_SFR), which is reasonably consistent well with corresponding recent observational results. The present study also predicts that inner regions of starburst disk galaxies have smaller alpha thus are more top-heavy (d alpha/d R ~ 0.07 kpc^{-1} for R < 5 kpc). The predicted radial alpha gradient can be tested against future observational studies of the alpha variation in star-forming galaxies.